Microcomputer Troubleshooting Guide

1. Visually inspect all parts and wires.  Sometimes a magnifying glass
helps.  Look for bent leads, short-circuited wires, cold solder joints,
etc.

2. Check power and grounds.  Do this with a VOM.  One lead is placed on
the ground at the power source and the other lead is touched to each power
pin.  Do this with the power on. When making checks always test on the
metal pins on the chip rather than on the circuit board.  Many times the
problem is the electrical connection between the circuit board and the
chip.

3. OPEN CIRCUIT TO DESTINATION. Check to see if there is continuity
between each address, data, or control line to their destinations. DO THIS
WITH THE POWER OFF.  Place one lead of the VOM on the CPU pin and the
other on the pin of each part that is supposed to be connected.  Set the
VOM to the 'beep' mode - this is usually indicated with a diode symbol.

4. SHORT CIRCUIT TO GROUND. Check to see if there is continuity between
each address, data, or control line and ground.  There should not be
continuity to ground.

5. SHORT CIRCUIT TO POWER. Check to see if there is continuity between
each address, data, or control line and power.  There should not be
continuity here as well.

6. SHORT CIRCUIT BETWEEN LINES. Check to see if there is continuity
between each address, data, or control lines and all of the other lines.  
Again there should not be any continuity.

7. Test each line with the power on using an oscilloscope.  This test
looks for dynamic clues as to why the computer is not working.  Check that
the clock is running.  Check that the RESET is in the correct state.  
Check all other inputs to the CPU, as well as inputs on each of the
devices on the bus.